ath79: add support to TrendNet TEW-673GRU

ath79: add support to TrendNet TEW-673GRU

Add support for the TrendNet TEW-673GRU to ath79.
This device was supported in 19.07.9 but was deprecated with ar71xx.
This is mostly a copy of D-Link DIR-825 B1.
Updates have been completed to enable factory.bin and sysupgrade.bin both.
Code improvements to DTS file and makefile.

Architecture   |  MIPS
Vendor         |  Qualcomm Atheros
bootloader     |  U-Boot
System-On-Chip |  AR7161 rev 2 (MIPS 24Kc V7.4)
CPU/Speed      |  24Kc V7.4 680 MHz
Flash-Chip     |  Macronix MX25L6405D
Flash size     |  8192 KiB
RAM Chip:      |  ProMOS V58C2256164SCI5 × 2
RAM size       |  64 MiB
Wireless       |  2 x Atheros AR922X 2.4GHz/5.0GHz 802.11abgn
Ethernet       |  RealTek RTL8366S Gigabit w/ port based vlan support
USB            |  Yes 2 x 2.0

Initial Flashing Process:
	1) Download 22.03 tew-673gru factory bin
	2) Flash 22.03 using TrendNet GUI

OpenWRT Upgrade Process
	3) Download 22.03 tew-673gru sysupgrade.bin
	4) Flash 22.03 using OpenWRT GUI

Signed-off-by: Korey Caro <korey.caro@gmail.com>
(cherry picked from commit 12cee869890853716ff1ee2dbd0a89c87a0ee544)